A first type of shuttle based automated warehouses uses one-directional shuttles to fetch and store pallets from and to the storage rows. To access the storage rows on all levels and in all positions within the aisle, the shuttle is put onto the appropriate storage row by a forklift. Alternatively, an automated stacker crane brings the shuttle to the appropriate level and position, i.e., the stacker crane moves the loads along the y- and x-direction. At ground level, the loads are then unloaded from the stacker crane for further distribution.
Automated stacker cranes are however expensive and typically only one crane at a time can move over the whole storage rack. Furthermore, due to the large weight of the stacker cranes, they have a high power consumption. These issues were solved by the introduction of two-directional shuttle systems. In such systems, on each level, shuttles move both along an aisle track, i.e., along the x-direction, and from the aisle track onto the storage tracks, i.e., along the y-direction. An elevator then exchanges the load between aisles on different levels. With the two-directional shuttle systems, different shuttles can move simultaneously on all levels.
In DE8210739U1 such a two directional shuttle system is disclosed. The two-directional movement is achieved by a first shuttle that moves along the aisle track and a second shuttle that moves along the storage track. This system is also referred to as a dual shuttle system. When moving along the aisle track, this second shuttle resides on top of the first shuttle. This way, the combination of the two one-directional shuttles achieves the two directional movement. A first disadvantage of this system is that it is expensive and complex as two separate compatible shuttles must be foreseen, each having their own batteries and motors. A second disadvantage is that, while the top shuttle resides in the storage track, the lower shuttle blocks the aisle track. Therefore, typically only one such shuttle system can be used per storage level.
This problem is overcome by the two-directional shuttle system as disclosed in CN203268812U. This single shuttle comprises two sets of wheels, one set for riding on the aisle tracks and one set for riding on the storage track. Each of the sets can be retracted when the other set is used. A disadvantage of this system is that the use of a single two-directional shuttle with two sets of wheels requires strict tolerances on the alignment between the aisle and storage tracks. The increased weight and thus cost of the storage rack in order to meet these stricter tolerances may however outweigh the initial advantage of avoiding the dual shuttle system.